This invention relates generally to electric motors and, more particularly, to a start winding cutout switch for a refrigerator compressor motor.
Electric motors typically include a start winding, a run winding and a magnetized rotor. The start winding is used to initiate rotation of the rotor. The run winding has a high inductive reactance relative to the start winding, so that the magnetic fields generated in the respective windings are out of phase with one another. The geometric time phase relationship between the magnetic fields causes the rotor to begin to rotate from a standstill condition when the windings are energized. Once the rotor has sufficient torque to attain its normal running speed, the start winding is "cut-out" of the motor circuit so that the geometrically spaced out-of-phase magnetic field generated by the start winding does not adversely affect motor operation. The start winding may be utilized as an auxiliary run winding after motor start-up by connecting a run capacitor in series with the start winding. Often, a run capacitor results in better motor efficiency and power factor.
A positive temperature coefficient resistor (PTCR) may be used to regulate the current flowing through the motor start winding. A PTCR is a temperature responsive resistor element that has a low resistance in a cool state, but a very high resistance when heated to an "anomaly temperature" or "Curie Temperature." When a PTCR is connected in series with a start winding, the low initial resistance in the cool state allows the start winding to draw a relatively large current to accomplish initial motor rotation. The current heats the PTCR until the Curie Temperature, and the very high resistance state, is reached. Very little current, therefore, flows into the start winding as the PTCR restricts or "chokes off" the current to the start winding to negligible levels. By selecting a PTCR having a Curie Temperature which is reached at approximately the same time that the motor running speed is achieved, a PTCR effectively and reliably regulates current flow into the start winding.
A PTCR, however, consumes 2-3 watts of power to maintain the high resistance state at the Curie Temperature. In light of stringent energy consumption standards, PTCR energy consumption is a factor in the efficiency rating of a compressor motor. PTCR energy consumption can be reduced, and the efficiency rating of a motor increased, by connecting a bimetal cutout switch in series with the PTCR. Current flowing through the switch heats a bimetal element therein, causing the switch to open and electrically disconnect the PTCR so that, except for the minute power loss in the wires and electrical connections, electrical power to the motor is dissipated only in the run and start windings. Thus, the energy efficiency rating of the motor is increased.
Depending on the characteristics of the bimetal element and the magnitude of the run current flowing through the bimetal cutout switch, additional heat may be needed to open the bimetal cutout switch element, or to maintain the bimetal cutout switch in the open position.
Accordingly, it would be desirable to provide additional heat to the cutout switch without compromising the energy efficiency of the motor.